Thermoplastic polymers, particularly TPO polymers such as polyethylene, polypropylene, ethylene/propylene/diene monomer (EPDM) rubber, etc., are particularly well suited for a wide variety of uses. However, because of their low stiffness and toughness properties relative to various engineering plastics, e.g., nylons, polysulfones, polycarbonates and the like, thermoplastic polymers are typically blended with one or more fillers to enhance these properties. Fillers that are planar in morphology, i.e., have a relatively high aspect ratio, tend to impart better stiffness and toughness to the TPO polymer than fillers with a nonplanar morphology, i.e., fillers with a low aspect ratio. Representative of the planar fillers are the organoclays and talc, and representative of the nonplanar fillers are glass beads.
The morphology of the planar fillers can be likened to a deck of cards with each card representing one plane of the filler. The planes or layers of the filler, e.g., an organoclay, are held together by ionic bonds to exchangeable cations that are located between the planes. If the cards or planes can be separated from one another, i.e., delaminated, the deck or filler will mix better with whatever it is blended, e.g., a TPO polymer. Planes are typically separated from one another by exchanging the natural occurring cation with a larger atom or molecule, and the greater the separation introduced between the planes, the better the mixing. If the spacing between the planes is increased but without sliding by one another to any significant degree (the spaces between the cards of the deck are widened (swollen) but the deck retains its overall shape), the filler is intercalated. If the spacing is increased to such an extent that the planes slide by one another (the cards of the deck are no longer positioned atop one another but rather scattered about but retaining some degree of overlap and thus the “deck” is shorter but much wider and/or longer), the filler is exfoliated. Exfoliated filler is typically a mix of “exfoliated” planes and intercalated planes, e.g., some of the “scattered” planes are “short” decks of swollen planes.
In the context of increasing the stiffness and/or toughness of a TPO polymer with planar filler, ideally the filler is exfoliated to the extent that the planes of the filler are reduced to a single layer, i.e., that none of the cards of the deck are atop one another. Because TPO polymers are nonpolar and many, if not all, planar fillers are polar, particularly the organoclays, exfoliating the filler in the presence of a TPO polymer, i.e., under conditions in which the non-exfoliated organoclay and TPO polymer are in blending contact with one another, to achieve full or homogeneous mixing of the polymer with the individual layers of the filler is very difficult. Efforts to date have not met with satisfactory success.
Montmorillonite is an organoclay, a multi-layer silicate. In its natural state, its layers are held together by ionic bonds to exchangeable cations located between the layers. As discussed by Kawasumi, et al. in Macromolecules, 1997, pp. 6333-6338, when such a silicate is blended with softened or melted polypropylene, the resulting shear forces are not sufficient to delaminate or exfoliate the silicate layers even when the cation is a quaternary ammonium ion because polypropylene is a relatively nonpolar polymer.
Usuki, et al., U.S. Pat. No. 5,973,053, solved this problem using two related but different approaches. The first approach (also described by Kawasumi, et al.) was to blend a quaternary ammonium-exchanged, multi-layered silicate with a maleic anhydride-modified polypropylene oligomer, and then add an unmodified polypropylene polymer. The maleic anhydride-modified polypropylene oligomer had sufficient polarity to exfoliate the silicate under the shear conditions of the blending process.
The second approach of Usuki, et al. was to blend a quaternary ammonium-exchanged, multi-layered silicate with a maleic anhydride modified polypropylene polymer. The maleic anhydride-modified polypropylene polymer had sufficient polarity to exfoliate the silicate under the shear conditions of the blending process.
Usuki, et al. pointed out that if a maleic anhydride-modified polypropylene oligomer was not used, then the average molecular weight of the maleic anhydride-modified polypropylene polymer should be limited to about 100,000.